This invention relates generally to electro-acoustic transducers and more particularly to split-ring cylindrical transducers.
As is known in the art, a transducer is a device that converts energy from one form to another. In underwater acoustic systems, transducers generally are used to provide an electrical output signal in response to an acoustic input which propagates through a body of water or an acoustic output into the body of water in response to an input electrical signal.
An underwater acoustic transducer designed primarily for producing an electrical output in response to an acoustic input is called a hydrophone. Hydrophones are typically designed to operate over broad frequency ranges and are also generally small in size relative to the wavelength of the highest intended operating frequency.
A transducer intended primarily for the generation of an acoustic output signal in response to an electrical input is generally referred to as a projector. Projector dimensions are typically of the same order of magnitude as the operating wavelength of the projector. Moreover, projectors are generally narrowband devices, particularly compared to hydrophones. Both hydrophone and projector transducers are widely employed in sonar systems used for submarine and surface-ship applications.
Projectors generally include a mechanically driven member such as a piston, tube, or cylinder and a driver. The driver is responsive to electrical energy and converts such energy into mechanical energy to drive the mechanically driven member. The driven member converts the mechanical energy into acoustic waves which propagate in the body of water. Most acoustic transducers have driver elements which use materials having either magnetostrictive or piezoelectric properties. Magnetostrictive materials change dimension in the presence of an applied magnetic field, whereas piezoelectric materials undergo mechanical deformation in the presence of an electrical field. Because ceramic materials used in piezoelectric ceramic drivers are generally incapable of supporting tensile stresses, which often leads to fracturing of the ceramic, it is generally required that the ceramic driver be placed in a condition of precompression or prestress. Precompression protects the ceramic element from tensile forces which are generally detrimental to ceramic piezoelectrics.
Because acoustic transducers are used in a wide variety of applications, their size, shape and mode of operation can be quite different.
A configuration for acoustic transducers used when light weight and small size is needed is the split-ring cylindrical transducer. A split-ring transducer generally includes a hollow tube having a longitudinal gap extending the length of the tube and a cylindrical ceramic driver having a longitudinal gap at an angular displacement, such that when the driver is disposed within the tube, the respective gaps are generally aligned. In one configuration, a cylindrical ceramic driver has electrodes on the inner and outer surfaces and is polarized in a manner such that when an alternating current is applied across the electrodes, the driver causes the hollow tube to expand and contract in the radial direction. Accordingly, the ceramic driver and the hoop-mode projector are said to operate in the radial mode. The "C" shaped projector vibrates similarly to a tuning fork with the motion of the centers of vibration on either side of the diametral plane of the split having a large displacement normal to the plane as compared to the point diametrically opposite the split, which has a relatively small displacement. The resonant frequency of the split-ring projector is a function of the diameter as well as the thickness and elasticity modulus of the tube and ceramic driver materials.
One problem with acoustic transducers, in general, is that with increasing ocean depth, hydrostatic pressure conditions increase to levels capable of fracturing the elements of the driver or collapsing the shell.
As is known by those of skill in the art, solutions to this problem include increasing the wall thickness of the shell, pressure compensating the transducer using inflatable bladders, or providing passive pretension to the shell.
Although a shell with an increased wall thickness provides a transducer capable of withstanding increased levels of hydrostatic pressure, the size of the transducer is correspondingly increased. However, this solution may not be acceptable in applications where the size of the transducers is required to be small. For example, sonar systems using transducers as sonobuoys are required to be small in order to facilitate their launching and deployment.
Pressure compensation of the transducer using bladders are generally only effective if the transducer is used at a particular ocean depth. Use of the transducer at a different depth where the hydrostatic pressure conditions are different would change the operating characteristics of the transducer. Active gas compensators, where the amount of pressure is variable, may be used in some applications, but are expensive and require recharging after each use.
The concept of passive pretension is accomplished such that the hydrostatic pressure does not provide stress to the driver elements, until the pressure overcomes the shell prestress. In the case of a split-ring transducer, prestress is generally applied to the cylindrical ceramic driver by using a split hollow tube having a diameter somewhat smaller than the diameter of the ceramic cylinder driver. The opposing arms or curved members of the tube are spread apart sufficiently for inserting the cylindrical ceramic element within the tube. Releasing the spreading forces on the opposing arms allows the tube to wrap itself around the ceramic driver and places the driver in compression. However, at very deep ocean depths, many of the materials used in fabricating transducer shells are unable to withstand the high hydrostatic pressure conditions that exist in these environments.
For example, a material suitable for use in fabricating split hollow tubes, aluminum 7075T6, typically yields at stress levels greater than 72,000 psi. For "A" size sonobuoy transducers limited to an outside diameter of 4.875 inches, an ocean depth of approximately 140 feet is sufficient for transferring the outside hydrostatic pressure load to the internal elements (i.e., electromechanical driver). This is well above ocean depths where transducers having limited size and good acoustic performance are required.